1. Field of the Invention
The present invention relates generally to methods and apparatus for use in well completion operations. More particularly, the invention relates to methods and apparatus for use in stage cementing a well bore.
2. Related Art
Cementing operations within well boreholes typically involve mixing a cement and water slurry and pumping the slurry down steel casing to particular points located in the borehole's annulus around the casing, in the open hole below, or in fractured formations. Multiple stage cementing has been developed to permit the annulus to be cemented in stages from the bottom of the well working upward. In multiple stage cementing, a cementer having cement ports is positioned proximate sections of casing or joints to be cemented within the borehole. Cement slurry is flowed through the bottom of the casing and up the annulus to the level of the cementer, thus closing off the bottom. Valves in the cementer are opened and cement slurry is then flowed through the cementer to a point further up the annulus. Stage cementing can be accomplished by employing such valved cementers at successive stages within the borehole.
Well tools which have been used for multiple stage cementing typically control the opening and closing of the cementer ports using sliding sleeves internally disposed within the housing of the well tool. Typically, two such sleeves are used, each of which is shear-pinned into an initially upper position such that the cementing ports of the tool are closed. It is common to open the cementing ports by use of a plug which is placed within the borehole and flowed down the casing until it is seated on the lower of the two sleeves. Fluid pressure within the casing is increased above the plug until the shear-pins holding the lower sleeve are sheared and the lower sleeve is moved downward to uncover the cementing ports. When a desired amount of cement has been released through the cementing ports these ports are closed by flowing a second plug down through the casing behind the cement until it is seated on the upper sleeve. Fluid pressure is increased within the casing behind the second plug until the shear-pins holding the upper sleeve are severed and the upper sleeve is moved down to close the cementing ports.
Unfortunately, a variety of problems can be encountered in the present method for opening and closing cementing ports. Occasionally, opening and closing the ports is extremely difficult since the sliding sleeves become clogged with cement or debris. Following the cementing operations, both the plugs and sliding sleeves must be removed from the tool by drilling them out. Since the plugs have been strongly pressured against the sliding sleeves, this drilling operation entails a significant degree of time and expense. It would be desirable, then, to provide a reliable means of opening and closing cementing ports which avoids the problems associated with the traditional plug and sliding sleeve arrangement.